This invention relates to an apparatus and method for intensifying cooling in the casting of metal objects, particularly in essentially vertical continuous casting carried out from bottom to top.
In continuous upward casting of a metal object, known for example from U.S. Pat. No. 3,746,077, the molten metal is drawn upwardly into a graphite nozzle where it is cooled, for example, by using a cooler 3 as shown in FIG. 1. The cooler 3 comprises a body of metal, such as copper, defining a vertical bore that contains the nozzle 1. The cooler 3 is surrounded by a jacket of thermal insulation material and is positioned relative to the melt so that the lower end of the nozzle 1 and the bottom part of the cooler 3 are below the free surface of the melt. A cooling agent, typically water, is conducted into the top of the cooler, via an inlet located in the vicinity of the outer wall of the cooler and flows downwards in an outer passage between the outer wall of the cooler and an intermediate pipe 4. From the bottom of the intermediate pipe the cooling agent is directed upwards in an inner passage towards an outlet while flowing in contact with the inner wall of the cooler and is discharged from the top of the cooler. A solidification front 2, where the molten metal turns solid, is formed near the bottom of the nozzle. The solidification front extends over a height H, from a level L1 to a level L2. It is apparent that the rate of removal of thermal energy from the nozzle 1 is at its highest essentially at the level of the solidification front 2, as shown in FIG. 1 by the arrows 5, because the metal, in the course of solidification, changes state and thus emits latent heat.
The cooler shown in FIG. 1 operates satisfactorily at relatively low casting speeds, because the thermal capacity of the cooler itself contributes substantially to removal of heat from the molten metal.
When using the prior art cooler shown in FIG. 1 for instance in the casting of wire, the casting is carried out at essentially high velocities. At the higher rate at which melt enters the nozzle, the flow of cooling agent is unable to remove heat from the cooler as fully as when the casting speed is lower, so that the temperature of the cooler is higher than when the cooler is operated at lower casting speeds. The increase in temperature of the cooler may lead to thermal expansion of the lower part of the cooler, which creates a gap in the threading between the nozzle and the cooler and leads to loss in efficiency in removal of heat from the nozzle. Moreover, the increase in the temperature of the cooler may result in an insulating steam bed being formed in the cooling agent, causing further loss in cooling efficiency. Therefore, the distance over which the metal entering the nozzle must pass in order for sufficient heat to be extracted for solidification to take place increases, with the result that the solidification front moves upwards and the height of the solidification front increases, as shown in FIG. 1A. The temperature of the cast object leaving the nozzle is substantially higher than when an object is cast at low speed While casting for instance copper wire at the rate of 6 m/min, the surface temperature of the wire may, after cooling, be over 500.degree. C. Such a high wire temperature may cause the wire to break off during casting so that the casting is interrupted and must be re-started. Restarting the casting operation is time consuming, so the rate of production of wire is reduced. Furthermore, if the wire breaks off during casting, the nozzle may be damaged, so that the re-starting may also involve replacement of the nozzle, adding to the cost of operation. If the length of wire that has been cast before the break occurred is quite short, it might not be usable and therefore have to be sent to remelt.
It is desirable that the height of the solidification front-be small and that the solidification front be formed near the bottom of the cooler, so that the cast object continues to be cooled by the action of the cooler over substantially the entire length of the nozzle that is within the cooler body.